Most polymeric materials possess viscoelastic characteristics that, when melt processed, may result in undesirable defects in the finished material. This is particularly evident when a polymer is melt processed above a critical shear rate and can cause the surface of the extrudate to exhibit melt defects (e.g., melt fracture, surface roughness, edge tear, sharkskin). A common melt defect is a rough surface on the extrudate, and is referred to as melt fracture. Melt fracture is primarily a function of the rheology of the polymer and the temperature and speed at which the polymer is processed. This phenomenon is particularly problematic with polymeric materials that contain interfering elements, including fillers. Adding fillers to polymeric systems increases the overall melt viscosity, thus making them more difficult to process and making melt defects more prevalent.
In order to alleviate the problems of melt defects it is well known to add processing aids to polymeric materials. The processing aids are believed to function by forming a dynamic coating on the processing equipment and producing interfacial slip between the processing equipment and the polymeric material. Interfacial slip in this instance is defined as the reduction of surface tension, and subsequently shear stress, between the polymer melt and the processing equipment. One known class of processing aids are fluoropolymers, such as homo and copolymers derived from vinylidene difluoride, hexafluoropropylene, and tetrafluoroethylene monomers. Fluoropolymers are known to improve processability and eliminate melt defects in thermoplastics compositions. However, it is also known that fluoropolymers can be less or even non-effective in the presence of additives or fillers having reactive sites, since such materials can have strong interactions with the fluoropolymer, thus preventing it from functioning properly. Thus, much higher levels of fluoropolymer must be utilized to eliminate melt defects from filled polymers, a solution that is often not cost-effective for the application.
International Patent Publication No. WO 2007/136552 discloses a polymer processing additive system which is particularly useful with filled polymers, which avoids the use of fluoropolymers and comprises a lubricant and a surfactant. Suitable lubricants for use with the additive system are said to include poly alkylene oxide oligomers and polymers, whereas suitable surfactants are said to include polyethylene-block-poly alkylene oxide oligomers.
In investigating the properties of fluorine-free polymer processing additive system disclosed in International Patent Publication No. WO 2007/136552, we have now found that the fluorine-free system operates by a different mechanism than conventional fluorine-containing polymer processing aids. Moreover, it has been found that by combining a fluorine-free polymer processing additive system with fluorine-containing processing aid, it is possible to achieve an unexpected and synergistic improvement in the effectiveness of the overall additive to improve processability and eliminate melt defects in thermoplastics compositions. This result not only offers the possibility of more effective processing aids requiring lower usage levels but also, in view of the generally lower cost of the fluorine-free system, should reduce overall polymer processing costs.
U.S. Patent Application Publication No. 2005/0101722, published May 12, 2005, discloses a melt processable composition comprising: (a) one or more thermoplastic hydrocarbon polymers; (b) a fluoropolymer processing aid having (i) poly(oxyalkylene) polymer; and (ii) a fluoropolymer having interpolymerized units of vinylidene fluoride and at least one other monomer wherein the vinylidene fluoride content of the fluoropolymer is greater than 65% by weight, and wherein said melt processable composition upon extrusion achieves an extrudate exhibiting no melt defects at a lower level of said fluoropolymer processing aid when compared to a standard processing aid system.